Typically marine vessels such as naval ships often slow down in rough seas so as to reduce seawater wave induced motions such as roll and pitch, because excessive amounts of such motion may seriously degrade combat readiness, adversely affect performance of on-board systems such as weapons and have other deleterious affects. Various methods have therefore been developed to reduce roll and pitch including use of active devices. Such active devices applied for example to fins, gyros, tanks and rudders often introduce cavitation, vibration and tip vortex problems at high travel speeds. It is therefore an important object of the present invention to provide active devices for inducing corrective motions on marine vessels during seawater travel under rough wave conditions, without introducing the problems heretofore experienced, and to also optimize fuel consumption during seawater travel at various speeds in calm water and low sea states while avoiding the cavitation problems heretofore experienced.
Pursuant to the present invention, a pair of flaps are pivotally mounted on the stern of a marine vessel between the port and starboard sides thereof, for displacement to positions with the upper surfaces thereof always exposed to air during vessel travel. The lower surfaces of such flaps when in deployed positions divert and smooth exit flow of the seawater from the stern between side plates along curved flow paths during vessel travel under rough sea conditions to hydrodynamically generate forces from the lower flap surfaces and the buttock of the vessel hull for pitch corrective purposes under in-phase displacement of the flaps and for roll corrective purposes under out-of-phase displacement of the flaps in opposite directions. During very lower speed travel the stern flaps are retracted above the water surface to avoid imposing drag penalty when corrective motion control is not needed. Flap control may also be utilized exclusively for corrective pitch motion involving use of a single flap element. The flap angular positions are also adjusted in accordance with ship speed variation to improve and optimize fuel savings during travel in calm water and low sea states. In accordance with a flap positioning control system to effect all of the foregoing aspects of the present invention, fuel savings and ship motion is improved during travel under different ship speeds and seawater conditions, including rough sea conditions as well as low sea states of the seawater.